The present invention relates generally to semiconductor devices and, more particularly, to a method for fabricating p-n and p-i-n junction-based devices utilizing III-V compound layers as well as the structures that result from the application of the method.
III-V compounds such as GaN, AlN, AlGaN, and InAlGaN have unique physical and electronic properties that make them ideal candidates for a variety of electronic and opto-electronic devices. In particular, these materials exhibit a direct band gap structure, high electric field breakdown, and high thermal conductivity. Additionally, materials such as InxAl1-xGaN can be used to cover a wide range of band gap energies, i.e., from 1.9 eV (where x equals 1) to 6.2 eV (where x equals 0). Unfortunately, although the III-V compounds are attractive for semiconductor applications due to their physical and electronic properties, until recently the development of devices based on III-V compounds has been limited by the lack of material with suitable conductivity, specifically p-type material.
In the late 1980""s, p-type GaN was grown, followed rapidly by the development of fabrication techniques for p-type AlGaN material. These materials were grown using metal organic chemical vapor deposition (MOCVD) techniques and, to a lesser extent, using molecular beam epitaxy (MBE) techniques. Since the development of p-type III-V material, a variety of semiconductor devices employing both p-n and p-i-n junctions have been demonstrated, including light emitting diodes (LEDs), laser diodes, and photo-detectors.
In the MOCVD technique, III-V compounds are grown from the vapor phase using metal organic gases as sources of the Group III metals. For example, typically trimethylaluminum (TMA) is used as the aluminum source and trimethylgallium (TMG) is used as the gallium source. Ammonia is usually used as the nitrogen source. In order to control the electrical conductivity of the grown material, electrically active impurities are introduced into the reaction chamber during material growth. Undoped III-V compounds normally exhibit n-type conductivity, the value of the n-type conductivity being controlled by the introduction of a silicon impurity in the form of silane gas into the reaction chamber during growth. In order to obtain p-type material using this technique, a magnesium impurity in the form of biscyclopentadienymagnesium is introduced into the reactor chamber during the growth cycle. As Mg doped material grown by MOCVD is highly resistive, a high temperature post-growth anneal in a nitrogen atmosphere is required in order to activate the p-type conductivity.
Although the MOCVD technique has proven adequate for a variety of commercial devices, it has a number of limitations that constrain the usefulness of this approach. First, due to the complexity of the various sources (e.g., trimethylaluminum, trimethylgallium, and biscyclopentiadienylmagnesium), the process can be very expensive and one which requires relatively complex equipment. Second, the MOCVD technique does not provide for a growth rate of greater than a few microns per hour, thus requiring long growth runs. The slow growth rate is especially problematic for device structures that require thick layers such as high voltage rectifier diodes that often have a base region thickness of approximately 30 microns. Third, n-type AlGaN layers grown by MOCVD are insulating if the concentration of AlN is high ( greater than 50 mol. %). Accordingly, the concentration of AlN in the III-V compound layers forming the p-n junction is limited. Fourth, in order to grow a high-quality III-V compound material on a substrate, the MOCVD technique typically requires the growth of a low temperature buffer layer in-between the substrate and III-V compound layer. Fifth, generally in order to obtain p-type III-V material using MOCVD techniques, a post-growth annealing step is required.
Hydride vapor phase epitaxy or HVPE is another technique that has been investigated for use in the fabrication of III-V compound materials. This technique offers advantages in growth rate, simplicity and cost as well as the ability to grow a III-V compound layer directly onto a substrate without the inclusion of a low temperature buffer layer. In this technique III-V compounds are epitaxially grown on heated substrates. The metals comprising the III-V layers are transported as gaseous metal halides to the reaction zone of the HVPE reactor. Accordingly, gallium and aluminum metals are used as source materials. Due to the high growth rates associated with this technique (i.e., up to 100 microns per hour), thick III-V compound layers can be grown.
The HVPE method is convenient for mass production of semiconductor devices due to its low cost, flexibility of growth conditions, and good reproducibility. Recently, significant progress has been achieved in HVPE growth of III-V compound semiconductor materials. AlGaN and AlN layers have been grown as well as AlGaN/GaN heterostructures using this technique. The AlGaN alloys grown by HVPE have been found to be electrically conductive up to 70 mol. % of AlN. Furthermore, since this technique does not require low temperature buffer layers, diodes with n-GaN/p-SiC heterojunctions have been fabricated with HVPE.
In order to fully utilize HVPE in the development and fabrication of III-V compound semiconductor devices, p-type layers must be produced. Conventional HVPE techniques have been unable, however, to grow such material. For example, if a magnesium acceptor impurity is added to a III-V layer grown utilizing conventional HVPE techniques, the resultant material is insulating (i.e., i-type) rather than being p-type. As a result, the potential of the HVPE technique for fabricating p-n or p-i-n junction devices has not been realized.
Accordingly, what is needed in the art is a method for fabricating p-type III-V compounds using the HVPE technique. The present invention provides such a method as well as a variety of structures realizable due to the ability to fabricate p-type III-V compounds using HVPE.
The present invention provides a method for fabricating p-type, i-type, and n-type III-V compound materials using HVPE techniques. If desired, these materials can be grown directly onto the surface of a substrate without the inclusion of a low temperature buffer layer. By growing multiple layers of differing conductivity, a variety of different devices can be fabricated using the invention.
According to the invention, one or more Group III sources are located within one or more source zones of a furnace where they are heated to a suitable temperature, typically between 750xc2x0 and 1050xc2x0 C. The Group III sources are in the form of pure metals, e.g., gallium and aluminum, and are preferably located on or within sapphire boats. A suitable substrate such as sapphire, silicon carbide, or gallium nitride is located within the growth zone of the furnace and heated to a temperature of between 800xc2x0 and 1200xc2x0 C., and preferably between 1000xc2x0 and 1100xc2x0 C. In order to achieve material growth, the Group III source(s) is exposed to a halide reactive gas, thereby causing the formation of one or more halide metal compounds (e.g., gallium chloride, aluminum trichloride, etc.). The halide metal compound(s) is transported to the growth zone as is a reactive gas such as ammonia containing the desired Group V element. As a result of the reaction between the reactive gas and the halide metal compound(s), a layer of single crystal epitaxial III-V compound material is grown. In order to achieve p-type conductivity, one or more acceptor impurity metals are transported to the growth zone during material growth, the transport rate achieving an acceptor concentration in the range of 1018 to 10 atoms per cubic centimeter, and preferably within the range of 1019 to 1020 atoms per cubic centimeter. Suitable acceptor impurity metals include magnesium, zinc, and magnesium-zinc alloys. Preferably the acceptor impurity metals are located on or within sapphire boats.
In at least one embodiment, the method of the invention is used to fabricate a simple p-n homojunction device. This device is comprised of an n-type III-V compound layer, e.g., GaN or AlGaN, grown directly onto the surface of a substrate without the inclusion of a low temperature buffer layer. A suitable p-type III-V compound layer is grown on the n-type layer, thus achieving the p-n homojunction. Contacts are deposited onto the structure, allowing for the conductivity of the selected substrate.
In at least one other embodiment, the method of the invention is used to fabricate a simple p-n heterojunction device, the heterojunction comprised of a GaN layer and an AlGaN layer. An example of a suitable device includes an n-type GaN layer and a p-type AlGaN layer.
In at least one other embodiment, the method of the invention is used to fabricate a double heterojunction device in which the light emitting layer is interposed between two layers, each of which has a wider band gap than the light emitting layer. As a result of the wide band gap layers, the device efficiently uses injected carriers. An example of a suitable device includes an n-type GaN layer between an n-type AlGaN layer and a p-type AlGaN layer.
In at least one other embodiment, the method of the invention is used to fabricate a device in which a p-n homojunction is interposed between layers of wide band gap material. The use of a p-n homojunction allows the device to achieve high recombination efficiencies while the surrounding wide band gap materials insure efficient use of injected carriers.
In at least one other embodiment, the method of the invention is used to fabricate a device utilizing a non-continuous quantum dot layer grown within the device""s p-n junction. The quantum dot layer is comprised of a plurality of quantum dot regions, each of which is typically between approximately 20 and 30 Angstroms per axis. The quantum dot layer is preferably comprised of AlxByInzGa1-x-y-zN, InGaN1-a-bPaAsb, or AlxByInzGa1-x-y-zN1-a-bP)aAsb. The p-n junction is preferably a homojunction consisting of layers of GaN or InGaN material. The p-n junction is preferably interposed between a pair of wide band gap materials.